1. Field of the Disclosure
Embodiments disclosed herein relate generally to apparatuses and methods related to the handling and processing of drilled solids from subsea wells. More particularly, embodiments disclosed herein relate to apparatuses and methods to promote proper operation of subsea pumps so as to minimize plugging process/mud return lines due to drill cuttings entrained therein. More particularly still, embodiments disclosed herein relate to methods and apparatuses to ensure continuous circulation of a wellbore by limiting particle size and reducing the number of larger particles.
2. Background Art
In off-shore drilling operations, drill cuttings that are generated in a well are pumped up from the drill bit to the surface utilizing rig mud pumps on a floating drilling vessel. The drill cuttings are carried back to the surface in a fluid medium called the drilling fluid or drilling mud.
Those skilled in the art and familiar with drilling technology will appreciate that the solids and cuttings generated during a drilling operation may span a wide spectrum of types, shapes and sizes. They range from the very fine clays (e.g., 0.2-2.0 .mu.m), to silt, sand, shale, limestone, claystone, sandstone, cement, and metal shavings generated during milling operations. Further, solids, such as cavings as well as sloughing shale from the well, may be generated, the sizes of which may vary anywhere from small pieces, two to five centimeters long, up to twenty-five centimeters in larger dimensions.
Along with the varying sizes of drill cuttings and solids, in certain areas in the world where the drilling operations are carried out (for example, in the Gulf of Mexico), the clay formations may be relatively younger and highly reactive. The clay formations may have a tendency to form a highly sticky mass of solids known in the drilling industry as “gumbo.” These types of hydrated clay formations may be difficult to drill because they have a tendency to stick to the drill bits and have been known to plug pipelines and flow lines, causing a variety of drilling problems. Further, these clay formations are commonly encountered in drilling operations; about 60-70% of all formations drilled are clays and shales.
Once the drill cuttings are generated at the drill bit, the cuttings travel upward through an annular space, first between the open borehole being drilled and a drill string, and then between the installed casing and the drill string, until they reach the sea floor. From the sea floor they may continue to the surface in the same drilling fluid medium through the annular space between the drill string and a wellhead, a blowout preventer, and a marine riser. At the surface, just below the rotary table, the drilling fluids along with various solids and/or the drill cuttings, may be diverted to a shaker that separates the drill cuttings from the drilling fluid. After passing through the shakers, the drilling fluid may circulate through further solids-processing equipment, for example, de-sanders and/or de-silters to separate sand and silt therefrom.
In drilling operations performed at large depths, the drill cuttings may be returned to the surface in a dual-gradient drilling operation. In a dual-gradient drilling operation, the returning drilling fluid does not enter the annulus of the marine riser extending from the sea floor to the drilling rig. Instead, it is diverted away from the wellbore at the sea floor by a rotating seal assembly, so that the marine riser is maintained full of seawater. The diverted drilling fluid along with the drill cuttings may then enter a pumping apparatus, for example, a mudlift pump. The solids laden mud entering pump chambers of the mudlift pump may be compressed and pumped up to about 265-275 liters (70-75 gallons) per stroke into the mud discharge piping and may maintain a flow rate up to about 3420 liters per minute (900 gpm). Thus, the mud discharge piping runs from the sea floor to the surface, thereby transporting the drill cuttings from the sea floor to the surface. In certain applications, the drill cuttings may be transported to a floating vessel for disposal.
Cuttings contained in drilling fluids passing through various components in a dual-gradient drilling operation raise significant concerns as compared to traditional subsea operations. In conventional subsea drilling operations, the drill cuttings are carried through the annulus of the riser. Common riser sizes used today may include sizes of about 54 cm (21 in) in diameter. Assuming a 65/8 inch size drill pipe, there may be an annular space, or radial gap between the riser and the drill pipe of about 17 cm (7 in) through which solids and drill cuttings may pass. Furthermore, larger sizes of solids are common in subsea drilling operations from caving, sloughing, broken cement plugs, and coiled masses of metal shavings known as “bird nests” resulting from metal milling operations.
Available literature discusses that in order for an opening to start plugging with solids, a minimum diameter of the solid particles should be at least equal to or greater than one-third a diameter of the hole through which the solid particles pass. This is the prerequisite to initiate bridging and to start plugging. Using this criteria, the minimum diameter of solids or the size of solids in the largest dimension that may easily pass through the present annulus may be about 6 cm (2.3 in). While this is a relatively large cutting size, a riser used in a conventional drilling operation described above would be able to pass such cuttings.
However, in a dual-gradient drilling operation, the drill cuttings and solids must additionally pass through mudlift equipment before returning to the drilling vessel. Mudlift pumps may have inlet and outlet lines too small to pass larger cuttings and solids therethrough. For example, inlet lines of a mudlift pump may have a diameter of about 15 cm (6 in), while outlet lines may have a diameter of about 11.5 cm (4.5 in). Additionally, internal chambers of the mudlift equipment may not be large enough to accommodate some larger solids. Based on the above criteria, and so as not to initiate bridging and cause plugging of the return lines or the mudlift pump chambers, the drill cuttings would have to be no larger than about 4 cm (1.5 in) along the largest dimension.
The above points out the importance of treating and handling solids in any type of drilling operation. Even in conventional drilling operations, with no hindrances posed in the return drilling fluid stream, the solids may have a tendency to cause severe problems. In dual-gradient drilling, these problems may be magnified with the added components through which the drilling fluid must pass. Previously, U.S. Pat. No. 6,102,673, issued to Mott, and incorporated fully herein by reference, discloses a dual-gradient drilling concept and mentions the utilization of a solids control system to restrict the size of the solids entering the system.
Additionally, solids returned to the surface and separated at the shale shaker may be important from a geological standpoint. The solids represent the various types of rock strata and geological formations that the drill bit has cut and drilled through. From the solids that are separated at the shale shaker, small samples may be examined at regular intervals by a geologist for study and analysis. This may be especially true in the case of an exploratory well where regular sample collection, tagging, study, and analysis are of paramount importance for the detection of hydrocarbons as well as for fossil study and other tasks that are deemed to be of geological importance. Those skilled in the art would appreciate the importance of preserving the integrity of the materials from the drilled formation as well as the need for its geological study.
Accordingly, there exists a need for a system to prevent a situation that may cause plugging and to facilitate the entry of the solids-laden drilling fluid into the pumping apparatus and through the return lines. Further, a system of bringing the drilled solids samples back to the surface with as little damage and as fast as possible would be well received in related industries.